1. Field of the Invention
This invention relates to an automatic exposure control device highly suited for a video camera, etc.
2. Description of the Related Art
The apparatuses of the kind automatically performing exposure control such as a video camera or the like are generally arranged to feedback control the iris of an optical system and an automatic gain control (AGC) circuit in such a way as to keep a video signal at a constant level.
FIG. 1 of the accompanying drawings shows in a block diagram the typical arrangement of the conventional automatic exposure control device used for a video camera. The illustration includes a photo-taking lens 1; an exposure control part 2 including an iris which is arranged to control the quantity of incident light coming through the photo-taking lens 1; an image sensor 3 which is a CCD or the like and is arranged to photo-electric convert information on an image formed on the image sensing plane thereof through the photo-taking lens 1 and to produce an image signal; an amplifier 4 which is disposed on the output side of the image sensor 3 and which includes a buffer amplifier and an AGC circuit or the like; and a signal processing circuit 5. The signal processing circuit 5 is arranged to convert a video signal output from the buffer amplifier 4 into a standard signal form of, for example, the NTSC system by performing a signal processing operation including a gamma correction process, a blanking process and a synchronizing signal adding process. The signal is thus converted into a signal form suited for a monitor display or a video recorder which is not shown and is output from a video output terminal. A signal level detecting circuit 6 is arranged to detect the level of the video signal output from the amplifier 4 and to feedback information on the detected signal level as a control signal to the exposure control part 2 and the AGC circuit, etc. included in the amplifier 4 in such a way as to keep the video signal at a predetermined level. This arrangement enables automatic exposure control to be carried out to keep the video signal at a desired level.
However, in the event of a great difference in luminance between an object to be photographed and the background thereof, this arrangement causes the image of the object to be darkened all over under a back-lighting condition or to be washed out under a normal lighting condition. The image then appears unnatural.
To solve this problem, a weighted light measuring method of measuring the light of the image sensing plane by attaching weight to a part thereof is employed. In accordance with the weighted light measuring method, a light measuring frame is set in the central portion of the image sensing plane. The inside of this frame is used as a light measuring area. The exposure control is performed always to make an exposure appositely to an object located within the light measuring area.
FIG. 2 shows in a block diagram the arrangement made in accordance with the above-stated weighted light measuring method. The arrangement differs from the device of FIG. 1 in the following point: In order to supply the signal level detecting circuit 6 with only a portion of the video signal corresponding to the inside of the light measuring frame located in the central portion of the image sensing plane, the arrangement of FIG. 2 includes a gate circuit 30 which is arranged to selectively gate the video signal to be supplied from the amplifier 4 to the signal level detecting circuit 6; and a gate pulse generating circuit 31 which is arranged to generate, in accordance with a synchronizing signal SYNC, a gate pulse for controlling the gate circuit 30 in such a way as to have only the signal of the inside of the light measuring frame allowed to pass through the gate circuit 30. That arrangement enables the automatic exposure control to be accomplished in such a way as to ensure an exposure apposite to the object""s image located within the light measuring frame without being affected by the background.
However, in the conventional device arranged to carry out the above-stated weighted light measuring method (hereinafter referred to as the frame-weighted light measuring method), the position of the light measuring area (or frame) is fixed on the image sensing plane. Besides, the exposure control is performed only for back-lighting compensation and wash-out compensation. Hence, the conventional arrangement for the frame-weighted light measurement has presented the following problems:
i) When the object""s image comes out of the light measuring frame due to a movement of the object relative to the camera, the brightness within the light measuring area changes. Exposure compensation is, therefore, effected according to this change. As a result, the brightness of the whole image sensing plane is caused to change. At the same time, the brightness of the object""s image also comes to change to give a very unnatural impression.
ii) A faulty operation might result from erroneous recognition of the object.
In view of these problems, it has been desired to have the exposure control accomplished appositely to any of varied conditions of the image sensing plane.
Further, the exposure control device for a video camera is arranged to adjust the amount of incident light by automatically controlling an iris according to the photographing conditions, particularly according to illuminance. The exposure control device detects the quantity of light through the level of a luminance signal included in a video signal and controls the iris in such a way as to make the luminance level unvarying.
Meanwhile, a tracing type light measuring method has been disclosed in U.S. patent application Ser. No. 264,204, filed on Oct. 28, 1988. In accordance with this method, the exposure control device is arranged to detect a difference in average video signal level between a video signal part obtained from the outside of a designated area of an image sensing plane and another video signal part obtained from the inside of the designated area; the position of the designated area is shifted in such a way as to bring the difference to a maximum difference value; and the quantity of light is detected on the basis of the video signal obtained from the inside of the movable designated (light measuring) area. The tracing type light measuring method is based on the assumption that there is a salient difference in level between a video signal for an object to be photographed and a video signal for the background of the object in general. Therefore, in cases where the video signal level difference between the object and the background is small, the small difference lowers the object tracing accuracy of an exposure control device arranged to operate in accordance with this method.
The present invention is directed to the solution of the above-stated problems of the prior art. It is therefore a first object of the invention to provide an exposure control device which is capable of performing exposure control always appositely to the condition of the image sensing plane of an image sensor.
It is a second object of the invention to provide an exposure control device which is capable of performing exposure control appositely to an object to be photographed even when the object moves.
It is a third object of the invention to provide an exposure control device which is arranged to automatically determine, through a video signal, whether the object to be photographed requires back-lighting compensation or wash-out compensation and to have the position of a light measuring area shifted to trace a dark object for the back-lighting compensation or to trace a white object for the wash-out compensation, so that exposure control can be accomplished always in an optimum manner.
Under this object, an automatic exposure control device arranged according to this invention as a preferred embodiment thereof comprises: light measuring area setting means for setting a light measuring area on an image sensing plane; discriminating means for discriminating a state of the image sensing plane on the basis of video signals corresponding to the inside and the outside of the light measuring area; exposure compensating means for making compensation for exposure conditions on the basis of the result of a discrimination made by the discriminating means; and tracing means for causing a set position of the light measuring area to trace an object to be photographed at least for a period during which the exposure compensating means is in operation.
It is a fourth object of the invention to provide an exposure control device which is capable of automatically carrying out back-lighting compensation or wash-out compensation by discriminating the state of an object and that of the background thereof from each other; is capable of keeping the camera in an apposite exposure state; and is capable of continuously performing adequate exposure control in real time by causing a light measuring area to trace the object even when the object moves.
It is a fifth object of the invention to provide an exposure control device which is capable of adequately performing object tracing type exposure control according to the state of an image sensing plane.
It is a sixth object of the invention to provide an exposure control device of an object tracing type which is capable of adequately operating according to the situation of a photographing object even in the event of a small difference in video signal level between the object and the background thereof.
It is a seventh object of the invention to provide an exposure control device which is capable of performing exposure control always appositely to the photographing conditions, the device being arranged to perform weighted light measurement for attaching weight to a photographing object by tracing the object only in a case where a brightness difference between the object and the background thereof is relatively large to permit a tracing action on the object; and to perform, in cases where the brightness difference between the object and its background is too small for tracing the object, either center-weighted or whole-plane average light measurement according to the degree of the brightness difference without tracing the object.
Under that object, an exposure control device arranged according to the invention as another preferred embodiment thereof comprises: first light measuring means for measuring a quantity of light according to an average level of a video signal obtained from the whole of an image sensing plane; second light measuring means for measuring a quantity of light by attaching weight to a video signal obtained from the inside of a light measuring area set in a predetermined position on the image sensing plane; and third light measuring means for measuring a quantity of light by attaching weight to a video signal obtained from the inside of the light measuring area while the light measuring area is moved to trace an object to be photographed. One of the three light measuring means is used by adaptively switching them from one over to another. When a luminance level difference between the video signals obtained from the inside and the outside of the light measuring area is above a predetermined value, the quantity of light is measured by the third light measuring means, so that the quantity of light can be measured appositely to a movement of the object. If the luminance level difference is a little smaller than the predetermined value, the center-weighted light measurement is performed by the second light measuring means. In a case where the level difference is much smaller than the predetermined value, the whole-plane average light measurement is performed by the first light measuring means.
The above and other objects and features of the invention will become apparent from the following detailed description of embodiments thereof taken in conjunction with the accompanying drawings.